Extraction of many samples for analysis requires the use of acid or base solvents. For example, acetic acid may be added during extraction of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinateddibenzofurans (PCDFs) from a sample comprising of fly ash pretreated with hydrochloric acid. In this case, the extraction vessel and associated plumbing is exposed to hydrochloric acid and acetic acid. Some samples that require pretreatment with concentrated acids, bases or other chemicals such as enzymes (hydrolysis, saponification, etc.) prior to extraction so that the analytes of interest are available for extraction by organic or aqueous solvent systems. For example, extraction/analysis of lipids in some food samples is difficult since the lipids are complexed with carbohydrate or proteins. Disintegration of the sample with an acid and heat hydrolyzes the proteins and starch and liberates the fat to allow its easy extraction to release bound lipids. Typical samples analyzed by the acid hydrolysis method are baked and cooked foods such as cereals, bread, cookies, chips, mayonnaise, cooked meals, meat products and cheese products. Association of official Analytical Chemists International (AOAC) method 966.06 describes acid hydrolysis step for these samples followed by a liquid-liquid extraction (LLE) method called the Mojonnier extraction method. The lipids are extracted away from the acid and other matrix components using a specific ether based solvent and then the solvent is evaporated leaving the lipids available for gravimetric analysis or further analysis by GC/GC-MS after converting the lipids to the fatty acid ester form.
Inert materials like glass are often used to contain samples extracted with aggressive solvents, e.g. concentrated acid or base, to ensure complete solubility of the analytes of interest. The nature of glass prevents these extractions from being performed under high temperatures and pressures, and so these extractions are typically done at or near atmospheric pressure and with only slightly elevated temperatures. As a result, the extractions performed under these conditions take longer periods of time than those typical pursued with a pressurized solvent extraction technique. For example, extractions such as these are often performed in test tubes, beakers or other similar laboratory ware.
Accelerated solvent extraction methods performed by the ASE® system sold by Dionex Corporation are an accepted solid liquid extraction (SLE) method useful for extraction of many types of analytes. (As used herein the term “ASE” refers to an accelerated solvent extraction method such as performed by the ASE® system and to the system itself.) This method is described in U.S. Pat. No. 5,843,311 (“the '311 patent”) and in EPA Method 3545. An automated system for performing the ASE method is described in U.S. Pat. No. 5,785,856. Other analytical methods include ones that pretreat samples with highly concentrated acids or bases (for example acid hydrolyzed samples with 8 M HCl).
Exposure to the acid in acid hydrolyzed samples can result in problems with existing instrumentation. For example, stainless steel components of the Dionex ASE® systems may be attacked by the acids particularly at the high temperatures (e.g. from 40° C. to 200° C.) employed during extraction, to blacken the stainless steel components is observed with the cell, tubing, filters and frits. In some extreme cases there is also clogging of the tubing and associated static valve failures. The above failures can affect robust system operation. Some of the above issues could be addressed in certain cases by cleaning the filters and frits by sonication or cleanup with concentrated nitric acid on a frequent basis or by replacing the components. These treatments can be cumbersome and could require aggressive reagents or they are expensive adding to the overall cost of the analysis. Similarly, when pursuing extraction with other aggressive solvents or sample conditions such as a caustic containing sample, the stainless steel components are affected.
A specific example of pretreatment prior to extraction is the pretreatment of food samples with either acid (hydrolysis) or base (saponification) prior to extraction with an organic solvent to determine lipid content. With hydrolysis, the samples are typically mixed with HCl (8 M) for 30-60 minutes at 70-80° C. After this, a couple of procedures can be followed. The liquid sample can undergo direct liquid-liquid extraction with a water immiscible solvent such as diethyl ether (Mojonnier Method AOAC Intl. Methods 989.05). Alternatively, the liquid sample can be mixed with combinations of diatomaceous earth and sand and then extracted by organic solvents (technique used by Foss and Buchi) or the liquid sample can be passed through a special filter paper (modified Rose-Gottlieb Method AOAC Intl. Methods 922.06, 932.06). Extraction of the filter paper provides the amount of fatty acids or lipids in the sample. In the case of saponification, the samples are usually acidified to protonate the fatty acids followed by liquid-liquid extraction with a water immiscible solvent to obtain the fatty acids (Schmid-Bondsynski-Ratzlaff Method).
The extraction of the sample, whether it is liquid-liquid extraction (LLE) or solid-liquid extraction (SLE), can be very time-consuming and can require large amounts of solvent. Very little automation exists (the Foss and Buchi apparatus are called automated Soxhlet extraction but still require 2-4 hours). Both Foss and Buchi use a mixture of diatomaceous earth (Celite) and sand to mix with the hydrolyzed samples. This mixture is then washed thoroughly with water to remove the HCl. After the water washing, the solid sample is placed in an automated Soxhlet extraction apparatus and extracted with hexane or petroleum ether for 2-4 hours. The construction of their apparatus is glass (inert), but the adsorbent used does not change the pH of the liquid extract or hydrozylate.
The use of materials such as zirconium, tantalum, hafnium, ceramics, Hastelloy, Inconel, etc. is known for use in inert valves, reactors and crucibles use for the digestion of materials. Companies such as Flowserve use titanium, Hastelloy, Inconel, zirconium and other metals and alloys in valves and valve parts. Tantaline puts a coating of tantalum on the surface of metallic parts to improve their corrosion resistance. Companies such as Evans offer zirconium or zirconium alloys as crucibles. There are also reports of using ceramics or “super alloys” for the lining of large scale reactor vessels.
The use of ion exchange materials for neutralizing acids and bases are known. (See U.S. Pat. Nos. 3,062,739 and 5,002,670.) Typically with acids anion exchange materials are used to remove the acid. In the lipid analysis case the use of anion exchange materials affects the recovery of the fatty acids since the acids tend to be retained by the anion exchange materials.